The present invention relates generally to systems for igniting fuel and, more particularly, to a single electrode spark ignition system for igniting fuel and to a system for controlling the gas flow to a burner using a pulse width modulated proportional valve. The present invention will be described primarily with regard to fuel-connected cooking appliances having electronic control circuitry. However, the present invention may be used in connection with virtually any apparatus or appliance which may need spark ignition or proportional gas flow control.
Gas cooking appliances have not extensively used microprocessor based electronic controls to control surface burners for two reasons: 1) the transient electrical pulses or voltage spikes from known single probe spark ignition systems may undesirably interfere with electronic circuits; and 2) a proportional valve may be needed which is adapted to precisely and cost effectively control the amount of gas in small incremental changes on a repeatable basis.
Fuel-connected cooking appliances may comprise a spark ignition system to ignite fuel at a burner. In known single electrode spark ignition systems for cooking appliances, fuel emanates from a burner which is grounded to the chassis of the cooking appliance. The chassis, however, may not be properly grounded. For example, the chassis of a cooking appliance may be resting on nonconductive plastic or rubber wheels, or the chassis may be resting on a nonconductive surface such as wood. In order to ignite the fuel, a voltage potential difference is generated between an electrode and the burner. The voltage potential difference may be in the range of 12,000 to 20,000 volts. Consequently, a 12,000 to 20,000 volt ignition spark is generated between the electrode and the burner. An ignition spark of this magnitude may cause transient electrical pulses or voltage spikes to undesirably interfere with the performance of electronic circuitry of the cooking appliance. For instance, the transient electrical pulses or voltage spikes may interfere with the performance of a microprocessor-based or microcontroller-based control circuit of a cooking appliance. The transient electrical pulses or voltage spikes may also reset a microprocessor power supply which typically operates at 5 volts. In addition, the transient electrical pulses or voltage spikes may damage components of electronic circuitry, may cause a microprocessor or microcontroller to incorrectly process information, and/or may cause electronic circuitry to lock-up or crash.
Due to the shortcomings of known single electrode spark ignition systems when used in conjunction with electronic circuitry, manufacturers of cooking appliances have instead used dual electrode spark ignition systems or hot surface ignitors to ignite cooking fuel. U.S. Pat. Nos. 5,003,960 and 5,033,449 disclose embodiments of a dual electrode spark ignition system. In a dual electrode spark ignition system, a spark is caused to jump from one electrode to another electrode, rather than from one electrode to chassis ground.
In order to prevent transient electrical pulses or voltage spikes from interfering with electronic circuitry, both electrodes of a dual electrode spark ignition system are heavily isolated from chassis ground and the electronic circuitry. For example, U.S. Pat. Nos. 5,003,960 and 5,033,449 utilize a ceramic insulating material to isolate the electrodes. Nevertheless, water, food, grease, or other conductive materials may gather on the insulating materials and short the electrodes to chassis ground and/or the electronic circuit. In addition, cracks may develop in the insulating material. As a result, water, food, grease, or other conductive materials may enter the cracks and short the electrodes to chassis ground and/or the electronic circuitry.
On the other hand, a hot surface ignitor may not interfere with the functions of a microprocessor or other electronic circuitry. For example, oven controls like those produced by Robertshaw of Long Beach, Calif. and supplied to companies such as the General Electric Company, Louisville, Ky., and the Whirlpool Corporation, Benton Harbor, Mich., utilize hot surface ignitors to avoid the problems presented by spark ignition systems. However, hot surface ignitors like those manufactured by Norton Company, Milton, N.H., may have three significant shortcomings. First of all, the ignitor elements may be made of silicon carbide or other similar fragile materials which may easily break or be damaged during shipment. Secondly, hot surface ignitors may have a high field failure rate due to the ignitor's elements burning out. Lastly, hot surface ignitors may cost approximately seven times more than a single electrode spark ignitor which is adapted for use in gas cooktops and the surface burners of ranges. Using hot surface ignitors, for example, on all four surface burners of gas cooktops or ranges would be too costly and too prone to field failures.
In light of the shortcomings of dual electrode spark ignition systems and hot surface ignitors, a need exists for a more reliable system for igniting fuel. Another need exists for a less costly system for igniting fuel. Still another need exists for a more durable system for igniting fuel. Finally, yet another need exists for a single electrode spark ignition system that does not damage or interfere with the performance of electronic circuitry.
Current range manufacturers use manual valves to control the flow rate of gas to surface burners. With such systems, users may determine the desired flow rate by visually looking at the flame height emanating from the burner and adjusting the manual valve to achieve the desired setting. To achieve desired cooking performance on a consistent basis it is necessary to cook recipes at the same temperature and for the same duration. It may be easier to accomplish cooking at the same temperature repeatedly on electric ranges due to the fact that the knob adjustment provides alphanumeric settings that can easily be remembered and can easily be set. This may not be as easy on known gas ranges because the user may have to estimate and remember how large the flame should be.
The nonlinear flow control of the prior art described in U.S. Pat. No. 4,930,488 requires a sensor to be placed in close proximity to the burner. The sensor provides a feedback loop to the control circuit so that it can alter the current applied to the grooved poppet to modulate the gas flow in response to signals from the sensor. Consequently, the nonlinear flow control system of U.S. Pat. No. 4,930,488 may be subject to irregular heat production, parts failure, associated assembly and inventory costs, and warranty costs.
In light of the shortcomings of the known art, a need exists for a system adapted to provide flow control without the need of a sensor or feedback loop. Another need exists for a flow control system having reduced risk of parts failure and lower assembly costs, inventory costs, and warranty costs. A need also exists for a system adapted to provide a consistent, precise, and repeatable flow of fuel to a burner. Still another need exists for a system whereby a user can precisely control the flow of fuel to a burner. Yet another need exists for a flow control system in which the poppet of a flow control valve reacts linearly in response to a change in the input command. In addition, a need exists to use a pulse width modulated control signal in conjunction with a magnetic field to obtain linear flow control. Finally, a need also exists for a linear flow control system having a timer function and an auto shut off function to enable a user to obtain consistent and repeatable preparation of recipes.
Preferred embodiments of the present invention satisfy some or all of the aforementioned needs. In particular, a preferred embodiment of the present invention is a gas appliance comprising a burner, a conduit, a control circuit, and a valve. The burner is adapted to burn a fuel and produce a flame. The conduit is connected to a source of the fuel and adapted to convey the fuel to the burner. The valve is interposed on the conduit between the source of fuel and the burner and is in electrical communication with the control circuit. The control circuit is adapted to provide a pulse width modulated signal to the valve whereby the valve provides a substantially linear flow rate of fuel to the burner.
In addition to the novel features and advantages mentioned above, other objects and advantages of the present invention will be readily apparent from the following descriptions of the drawings and preferred embodiments.